Air brake systems are commonly used for tractor vehicles that tow trailers. Such systems include a tractor protection valve that controls air supply to trailer brakes. The tractor protection valve receives an air signal from a park brake control valve that is typically mounted to a dash in the tractor. The air signal from the park brake control valve can be used to apply or release trailer spring brakes, sometimes referred to as parking or emergency brakes, and can be used to charge or re-supply trailer reservoirs, which typically comprise primary and secondary air sources. The tractor protection valve also receives air signals from a service brake foot valve or trailer hand control valve to apply or release trailer service brakes. Some tractor protection valves include check valves that blend air from the primary, secondary, and hand control air sources that are used to apply the trailer service brakes.
If there is insufficient air pressure in the parking brake or re-supply portion of the tractor protection valve, the tractor protection valve is configured to prevent air from being delivered through the service brake portion. This prevents compounding of the trailer spring brakes and protects a tractor air supply by blocking a loss of air pressure out of a service control line in the event that a connection line between the tractor and trailer is inadvertently disconnected. Compounding refers to the undesirable condition where parking and service brakes are applied simultaneously.
One disadvantage with current configurations is the amount of time it takes to apply and release trailer parking brakes. If a line from a park brake control port to a supply port of the tractor protection valve is long, or if it is somehow otherwise restricted due to routing through a tractor undercarriage, park brake apply and release times are adversely affected. Likewise, if service brakes lines to the tractor protection valve are long or restricted, then service brake apply and release times are adversely affected.
Further, some brake systems use a stability or roll-control sensor to apply trailer brakes without driver intervention in response to a sensed stability event. A solenoid valve assembly can be connected to the tractor protection valve to accomplish this function. These additional connection lines increase system costs.
Thus, there is a need for a tractor protection valve configuration that provides faster apply and release times for trailer service and parking brakes, and which reduces costs by eliminating the number of air connections to separate components.